The invention relates to a drive device, which is to be used in particular for the closing unit or the injection unit or the ejectors of a plastics injection molding machine, with a drive element (10) which is axially moveable by an electric motor and with a hydraulic unit (12) which, by moving the drive element (10), is moveable in same direction as the latter.
Within the closing unit of a plastics injection molding machine, the drive device moves the movable platen of the machine. Such a drive device has to meet two different important requirements. On the one hand, it is to move the platen as quickly as possible for closing and opening the mold, in order to keep down the cycle time for producing a molding. On the other hand, it is to be capable of keeping the platen, and consequently the entire mold, closed with great force against the high injection pressure. Therefore, on the one hand positioning movements are to be performed at high speed, on the other hand great forces are to be exerted without significant movement. Such requirements may arise not only in the case of the closing unit but also in the case of the ejectors or the injection unit of a plastics injection molding machine. For example, during the injection of polymer into the mold, the placticizing screw is moved at relatively high speed in the direction of the mold, until the mold is completely filled with polymer. If, following this, the polymer melt located in the mold is subjected to a so-called holding pressure, the drive must provide a great force without significant movement of the plasticizing screw.
U.S. Pat. No. 4,030,299 discloses a purely hydraulic drive for the movable platen of a plastics injection molding machine which also includes a hydraulic force multiplier. This has a movable piston of a small effective surface area, a further movable piston of a large effective surface area and a cylinder, which together with the pistons encloses a pressure chamber filled with a pressure fluid. The cylinder is fixedly arranged on the frame of the injection molding machine. The drive also includes hydraulic cylinders, which move the movable platen for closing and opening the mold. In the opened state of the mold, the volume of the pressure chamber of the hydraulic force multiplier is minimal. If the movable platen is then moved by the hydraulic cylinders in the sense of closing the mold, the large piston of the hydraulic force multiplier is taken along, with the volume of the pressure chamber of the hydraulic force multiplier increasing and pressure medium flowing out of a tank into the pressure chamber via a replenishing valve. Following this, the small piston of the hydraulic force multiplier is moved into the pressure chamber and thereby produces a high pressure, which brings about a great closing force by acting via the large effective surface area of the large piston. The small piston is hydraulically moved by supplying pressure fluid. Consequently, in the case of the drive device according to U.S. Pat. No. 4,030,299, various hydraulic drive components are present for the positioning movement of the movable platen and for the exertion of a great force. During the positioning movements of the platen, considerable pressure fluid flows back and forth between the pressure chamber and the tank, which necessitates correspondingly large valves and fluid channels.
A drive device with the features of the introductory paragraph is known from DE 41 11 594 A1. In the case of this drive device, a hydraulic cylinder with a large effective surface area is fixedly connected to the movable platen. The unit comprising the movable platen and the hydraulic cylinder can be moved by an electric motor via a gear mechanism, which comprises a lifting spindle and a spindle nut, to obtain rapid closing and opening of the mold. The great closing force is provided by pressure exertion of the hydraulic cylinder which can be moved with the platen. In this case, the entire force of reaction is diverted via the spindle and the spindle nut to the machine frame. Apart from the components of the electric drive, the plastics injection molding machine according to DE 41 11 594 A1 is also equipped with a complete hydraulic system, including an oil tank, pump, valves and hydraulic cylinders.